"They've got a hair-letting down. I think we call it in England ... a knees up," Dickie Arbiter, former press secretary to the queen, said on "Good Morning America" today. "That's exactly what it's going to be. It's going to be a sit-down dinner and then a band and disco."

In a royal wedding "first," William and Middleton have elected to have an evening party for an intimate circle of 300 family and friends.

The evening, hosted by Prince Charles, will include drinks, a sit-down dinner, speeches and dancing.

Three rooms on the west side of the palace have reportedly been set aside for the occasion; notably the gilded red and gold throne room, traditionally used for visiting heads of state.

"It's going to be what William called the chill-out room, where the people are fairly tired, they can go in there and relax," Arbiter said.

Couches have reportedly been brought into the throne room for the occasion and drinks will be served.

Royal Wedding After-Party, Disco Balls Included

A larger room, the ball room, will also be used quite possibly as a disco. Measuring 120 by 60 feet, it is the largest room in the palace.

Middleton's sister, Pippa Middleton, is an event planner herself. She has reportedly taken it upon herself to install several actual disco balls in the palace for the party, much to the apparent chagrin of some palace staffers.

"I'm not sure I want to see glitter balls in the palace," ABC News contributor and royals expert Katie Nicholl said. "There's more than enough bling in there."

William's decision to have such a party comes as no surprise to royal insiders. They said it's symptomatic of a new generation of royals poised to inherit the throne in the not-so-distant future.

"That's the sort of character that William is. He's very easy. He's very relaxed," Arbiter said. "He doesn't have to be as formal as his father was."

What a couple usually does after the wedding, Nicholl said, is go straight to their honeymoon. "This is changing the guard a little bit," she said.

Watch a special "20/20" Thursday at 8 p.m. ET for a behind-the-scenes look at the life that awaits Kate Middleton and join us again at 4 a.m. Friday for ABC News' live coverage of the Royal Wedding with Diane Sawyer and Barbara Walters.

Abba Tunes Expected to Play at After-Party

Among the tunes expected to fill the palace Friday night: Abba, a reported favorite of Middleton's.

The queen will reportedly attend the dinner, then retire for the evening soon afterward. "She's already said she'll let the youngsters get on with it. And they will get on with it," Arbiter said.

Nicholl added that Prince William has argued that he is not the heir but the heir apparent, which gives him more room for new ideas. "So if he wants disco balls, if he wants Abba all night, who's going to argue?" she asked.

The plans, including the installation of those disco balls, has apparently raised the eyebrows of palace staff. Nicholl said she thinks the public will understand what the young couple is doing, even if it's non-traditional.

"Let's face it, when William is eventually king, there are not going to be opportunities like this. He has this chance now, and I don't think anyone would begrudge him," Nicholl said.

Meanwhile, the queen and Prince Philip will be heading to Windsor Castle for the night, leaving the newlyweds and their friends to party all night.

"I think the fact that her majesty the queen is prepared to leave the palace for one night, and one night only, says that it also has her blessing," Nicholl said.

Even if the queen changes her mind and decides to stay at Buckingham Palace Friday night, Arbiter said, she won't have a problem falling asleep. A distance equal to an entire city block separates her bedroom from the palace's ballroom where the party will take place.